The main aim of the proposed psychopharmacological studies is to determine the effects of stimulant medication (methylphenidate [MPH]) on specific cognitive (particularly inhibitory) processes in children with ADHD. Inhibitory control processes determine the ability to stop and change current thoughts and actions. An important aspect of the proposal is its goal to look at certain cognitive processes as separable from other dimensions of behavior that are often made more normal using MPH therapy. Four interrelated studies are proposed that are based on a novel theory and methodology that emphasize the efficiency of inhibitory control and phonological processing. The specific aims are to: 1) identify the cognitive mechanisms for the inhibitory control deficits found in ADHD children; 2) examine the basis of the comorbidity of ADHD and dyslexia (or reading disorders) by means of a test for double dissociation of two contrasting cognitive domains (inhibitory control, phonological processing); 3) determine whether DSM-VI subtypes of ADHD can be differentiated on the basis of inhibitory control mechanisms; 4) determine whether stimulant medication produces beneficial or deleterious effects on the cognitive mechanisms of ADHD children; and 5) evaluate the utility of individual growth curve models for predicting stimulant effects on inhibitory control processes and overt behavior at the level of the individual and from subject characteristics such as age, concurrent disorders, medication history, and so on. Subjects will be children with ADHD and matched contrasts without ADHD.